It’s ironic that the very site in question, despite claiming XHTML compliance, is served as text/html instead of application/xhtml+xml, so the browser will never parse it as XML.
To quote [0]:
> All those “Valid XHTML 1.0!” links on the web are really saying “Invalid HTML 4.01!”.
Although the article is 20 years old now, so these days it’s actually HTML5.
Edit: Checked the other member sites. Only two are served as application/xhtml+xml.
[0]: https://webkit.org/blog/68/understanding-html-xml-and-xhtml/
>>these days it’s actually HTML5.
There is no HTML5. It's just a buzzword. https://html.spec.whatwg.org/dev/introduction.html#is-this-h...?
And this makes the XML prolog invalid, because it's invalid to have it in HTML.
Not having it is XHTML compliant though, so it could just be removed.